Zurich University neuropsychologists were so intrigued by the case of E.S. -- a 27-year-old professional musician whose full name has been withheld -- that they recruited her for a year-long inquiry.

They say she is the world's most extreme known case of synaesthesia, the phenomenon whereby hearing music triggers a response in other sensory organs.

E.S. sees colours when she hears a tone, with for instance an F sharp causing her to see violet while a C makes her see red, quite literally.

Even more remarkable is that she also gets a taste on her tongue according to the note she hears.

A tone interval of a minor second induces sourness, while a major second leaves a bitter taste.

A minor third is salty, while a major third is sweet.

Other tastes, according to the tone, are of "pure water", cream (either full or low-fat, depending on the note), "disgust" and also of mown grass.

To provide an objective test, the scientists applied one of four different-tasting solutions (sour, bitter, salty and sweet) to her tongue and then asked her to press a button on a computer keyboard corresponding to four relevant tones.

She responded with perfect accuracy and much faster than five musicians, recruited for the same test, who do not have her synaesthesic gifts.

E.S.'s "extraordinary" synaesthesia has probably been a boon in her career by attuning her to the right pitch, the researchers say.

The study, led by Lutz Jaencke, appears on Thursday in the British weekly science journal Nature.